The present invention relates to a catalyst comprising halide-containing alkali tungstates for the synthesis of alkylmercaptans from alkanols and hydrogen sulfide, and to a process for the preparation of this catalyst.
The term “alkali” is to be understood as meaning bonded alkali metals according to the periodic system of the elements, it also being possible for the tungstates to contain two or more different bonded alkali metals. The term “halide” is to be understood as meaning the bonded halogens of the periodic system of the elements, it also being possible for the alkali tungstates according to the invention to contain two or more different halides.
Among the alkylmercaptans, methylmercaptan in particular is an industrially important intermediate, for example for the synthesis of methionine and also for the synthesis of dimethyl sulfoxide and dimethylsulfone. Nowadays, it is prepared predominantly from methanol and hydrogen sulfide by reaction on a catalyst of aluminum oxide. The synthesis of methylmercaptan is usually carried out in the gas phase at temperatures of from 300 to 500° C. and at pressures of from 1 to 25 bar.
As well as containing the methylmercaptan that is formed, the reaction mixture contains unreacted starting materials and secondary products, such as, for example, dimethyl sulfide and dimethyl ether, as well as gases that are inert in terms of the reaction, such as, for example, methane, carbon monoxide, hydrogen and nitrogen. The methylmercaptan that forms is separated off from this reaction mixture.
For the economy of the process, it is necessary for the yield in the catalytic reaction of methanol and hydrogen sulfide to methylmercaptan to be as high as possible, in order to keep the outlay in terms of the separation of the resulting methylmercaptan from the reaction mixture as low as possible. In particular, the energy outlay for cooling the reaction gas mixture in order to condense the methylmercaptan represents a major cost factor here.
In order to increase the activity and selectivity, potassium tungstate or caesium tungstate is usually added to aluminum oxide as support. The tungstate is usually used in amounts of up to 25 wt. %, based on the total weight of the catalyst. An improvement in the activity and selectivity is also obtained by increasing the molar ratio of hydrogen sulfide to methanol. Molar ratios of from 1 to 10 are conventionally used.
However, a high molar ratio also means a high excess of hydrogen sulfide in the reaction mixture and hence the necessity of circulating large amounts of gas. In order to reduce the energy outlay required therefore, the ratio of hydrogen sulfide to methanol should therefore differ only slightly from 1.
U.S. Pat. No. 2,820,062 relates to a process for the preparation of organic thiols, in which there is used a catalyst of active aluminum oxide to which potassium tungstate has been added in an amount of from 1.5 to 15 wt. %, based on the weight of the catalyst. With this catalyst, good activities and selectivities are achieved at reaction temperatures of 400° C. and molar ratios of 2. This U.S. patent specification mentions various possibilities for incorporating the potassium tungstate into the aluminum oxide. For example, impregnation processes, coprecipitations and pure mixtures are said to be usable. The actual preparation of the catalyst is accorded little importance for the economy of the synthesis process of methylmercaptan. EP 0 832 687 B1 describes the advantages of the use of caesium tungstate (Cs2WO4) as promoter instead of potassium tungstate (K2WO4). For example, increased activity with simultaneous good selectivity can be achieved by the use of caesium tungstate.
By increasing the caesium tungstate concentration to up to 40 wt. %, the selectivity in respect of methylmercaptan can be increased to up to 92% without the activity being disproportionately impaired.
The general opinion is that the best selectivity is achieved with catalysts in which the alkali/tungstate ratio is 2:1 (A. V. Mashkina et al., React. Kinet. Catal. Lett., Vol. 36, No. 1, 159-164 (1988)).
The object of the present invention is to provide a catalyst and a process for its preparation, which catalyst is distinguished at low molar ratios of hydrogen sulfide to methanol by improved activity and selectivity compared with the known catalysts and accordingly results in a better yield and greater economy of the process.